This invention relates to semiconductor devices, particularly to those of the class employing nitrides or nitride-based compounds as semiconductors, as typified by metal semiconductor field-effect transistors (MESFETs) and high electron mobility transistors (HEMTs), and more particularly to such semiconductor devices of reduced leakage current. The invention also specifically pertains to a method of making such semiconductor devices.
The gallium-nitride-based semiconductor devices were known and used extensively which had the gallium-nitride-based semiconductor region formed on a sapphire substrate via a buffer region. The sapphire substrate had, however, a weakness of being difficult of dicing for its hardness, in addition to being expensive. These shortcomings of the sapphire substrate are absent from the substrate of silicon or silicon compound suggested by Japanese Unexamined Patent Publication No. 2003-59948. Being higher in heat conductivity than sapphire, the silicon or silicon-based substrate better liberates the heat that generates during operation of the semiconductor device, resulting in improvement in gain and other performance characteristics.
As taught also by the unexamined patent application above, a multilayered buffer region is interposed between the silicon or silicon-based substrate and the nitride semiconductor region constituting the primary working part of the semiconductor device. The buffer region is in the form of alternations of two different kinds of buffer layers such for instance as AlxGa1-xN, where the suffix x is a numeral grater than zero and equal to or less than one, and GaN.
The multilayered buffer region offers some definite advantages. First, being intermediate in lattice constant between the substrate and the gallium-nitride-based semiconductor region, the buffer region well conforms to the crystal orientation of the substrate. As the crystal orientation of the substrate is favorably conveyed to the buffer region, so it is to the semiconductor region. The result is improvement in the flatness of the semiconductor region, which in turn leads to better electrical characteristics of the semiconductor device.
Second, constituted of alternating AlxGa1-xN and GaN as above, the buffer region is capable of crystal growth at higher temperatures than a buffer region consisting solely of either of these compounds. There is therefore little or no fear of the multilayered buffer region from becoming amorphous. The main semiconductor region, grown epitaxially on the multilayered buffer region, is to suffer less crystal defects, with a consequent reduction of leakage current.
However, when formed on a silicon or silicon compound substrate, which is low in electrical resistance, the multilayered buffer region has proved to possess its own drawbacks. Leakage current was easy to flow into the multilayered buffer region in a prior art device such as a HEMT in which the nitride-based semiconductor region was formed via the multilayered buffer region on the low resistance substrate. Even when no voltage was impressed to the gate of the HEMT, there was current leakage between drain and source, either via the multilayered buffer region or via the buffer region and low resistance substrate. The current leakage is of course a cause for power loss or a deterioration of performance characteristics. Further, in cases where the voltage-withstanding capability of the device is determined on the basis of the magnitude of leakage current, that capability was evaluated to be very low.
An obvious solution to the current leakage problem might seem to substitute aluminum nitride layers, which are more electrically insulating, for the AlxGa1-xN layers of the multilayered buffer region, and to make the aluminum nitride layers thicker. Such aluminum nitride layers would give rise both to a lattice mismatch with the neighboring gallium nitride layers and to a difference in coefficient of thermal expansion, such that the aluminum nitride layers might be subjected to tensile stresses. The result would be the cracking of the multilayered buffer region or of the main semiconductor region formed thereon.